Mechanical couplings for joining pipe elements in end to end relation often use ring seals bridging the pipe elements, the ring seals being compressed between coupling segments and the pipe elements to ensure fluid tightness of the mechanical joint. It can be a challenge to position seals around pipe elements when the ring seals are made of non-elastomeric materials, such as thermoplastic or polymeric composites as well as metals, composites and combinations thereof suitable for extreme (high and low) temperature service where traditional elastomeric materials are inappropriate. The ring seals made from such compounds or metals tend to be relatively stiff, with low elasticity and low resilience, especially when compared with seals made from elastomeric materials such as artificial and natural rubber compounds.
The outer diameter of any commercial pipe of a particular schedule will vary about a nominal value, both greater and smaller, within an acceptable manufacturing tolerance range. Likewise, there are manufacturing tolerances that create variability in the diameter of the surfaces of the ring seal that interfaces with the pipe elements, as well as the inner surfaces of the coupling housings that engage with the seal and create the compressive force necessary to create a fluid-tight joint. In order to assure that such joints are fluid-tight, practical embodiments of mechanical couplings are often designed with significant radial compressive deformation of the seal so that there is sufficient sealing force over the entire combined range of manufacturing tolerances of the seal, coupling and pipe elements, especially in the condition where the outer diameter of the pipe element is at the lower limit of its tolerance range and the inner diameter of the seal and coupling are at the outer limits of their respective tolerance ranges. In order to accommodate that significant radial compressive deformation, a seal material is chosen that is able to tolerate that deformation without warping or buckling, while also remaining sufficiently resilient and elastic. Ideally, such materials will have a relatively low modulus of elasticity, which is the relationship between the deformation of the material and the force required to create that deformation, ensuring that the high radial compressive deformation that mechanical coupling seals require can be applied through common means, such as with bolts, and that the couplings do not need to be made of impractically heavy sections and strong materials in order to tolerate those forces. Such materials are often highly elastic, meaning that they can undergo significant total deformation before the material is itself damaged. Therefore, highly elastic materials with a relatively low modulus of elasticity, such as elastomers, are commonly used in such circumstances due to their ability to accommodate that high degree of radial compressive deformation with moderate applied forces, without damage to the material, and without the seal distorting or warping in a manner that would compromise its effectiveness. However, such seal materials have drawbacks, such as limited ability to resist high or low temperature environments or certain chemicals. Alternative seal materials, such as metals, thermoplastics, fluoropolymers, or composite materials, offer improved performance with a wider variety of fluids and in those high or low temperature environments, but such materials often have a significantly higher modulus of elasticity combined with lower elasticity, resilience, and ability to tolerate the deformation needed to ensure effective sealing over the combined tolerance ranges of the seal, coupling, and pipe elements without damage to the material. The high forces needed to exert the required radial compression on these alternative materials may not be readily achieved without, for instance, excessive bolt torque, due to the higher modulus of elasticity of those alternative materials. Even where such high forces can be applied, seals made from such alternative materials may not readily accommodate those forces, and may warp or buckle, compromising the effectiveness of the seal. Coupling housings may need to be made stiffer and heavier in order to both accommodate those high forces and attempt to prevent the seals from warping or buckling enough to compromise the effectiveness of the seal. Those alternative materials' relatively low elasticity may not allow them to tolerate the high deformation required of mechanical coupling seals without damage to the material itself. One way of attempting to overcome the challenges associated with such alternative materials in mechanical couplings that must remain fluid tight over a range of combined manufacturing tolerances is to attempt to reduce the effect of those combined tolerances by precisely machining the coupling, seal, and pipe elements. Another method is to design the seal to have a maximum inner diameter that is smaller than the smallest acceptable outer diameter of the pipe elements for which the seal is designed as this initial interference can reduce the amount of required radial compressive deformation. However, precision machining is costly, often impractical to perform in the field, and limits the types of pipe elements that can be used to those which are machined. Further, it is difficult for a technician to install a seal made of these alternative materials over a pipe element having a larger outer diameter than the seal inner diameter. Difficulties arise when significant force is necessary to position a seal around a pipe element due to the initial interference combined with these materials' relatively high modulus of elasticity. Such force may not be readily applied manually, and may result in damage to the sealing surfaces or require special equipment and techniques to effect installation, resulting in less practical and reliable jointing. There is clearly an opportunity to improve sealing of joints provided by mechanical couplings, especially for wider chemical compatibility and for high- or low-temperature applications that employ seals made of alternative materials that have a relatively high modulus of elasticity, lower elasticity, and lower resilience.